The present invention relates to a method of and apparatus for treating pulp, such as a suspension of cellulosic fibrous material, with hydrocyclone cleaners. Pulp is treated with hydrocyclone cleaners in a centrifugal cleaning plant for separating impurities from the pulp.
Hydrocyclone cleaners are commonly used in the pulp and paper industry for cleaning fiber suspensions. The purpose of conventional hydrocyclone cleaners is to separate sand and other heavy fractions, as well as other impurity particles originating from wood, such as bark, and to reduce the shive-content of the pulp.
In a conventional hydrocyclone cleaner, material in the pulp that is heavier than fiber and water is separated into reject faction of the pulp. The feed pulp is a stream of pulp flowing into the hydrocyclone cleaner. The feed pulp is divided into two fractions, which are an accept faction (or just accept) that is taken out from the top of the cleaner and a reject faction (or just reject) that is taken out from the bottom of the cleaner. The feed pulp is thickened into the reject, whereby the reject faction is at a higher consistency than the feed pulp and the accept faction is at a lower consistency than the feed pulp. Consistency in a pulp suspension may be viewed as the mass or weight percentage of pulp fibers in the suspension. In general, consistency is the mass or weight percentage of particles in a suspension.
In a hydrocyclone cleaner, the pulp is fed at a low consistency to a conical vortex chamber wherein pressure energy in the pulp is converted to a rotating motion in the chamber. In a hydrocyclone cleaner, the separation of particles from fibers takes place under the influence of a centrifugal acceleration field resulting from the rotating motion. A precondition for the separation of particles in the conical vortex chamber is that the particles must move in relation to each other. It is known that this is possible only at low pulp consistencies; otherwise the fiber network in the pulp binds small impurities to itself and no separation occurs between the fibers and impurities in the vortex chamber. The efficiency of the vortex chamber in separating particles to be removed is dependent on the size, shape and density of the particles, and of the control variables such as the inlet velocity, density, and the pressure difference between the feed and the accept.
In reverse centrifugal cleaning, water and material lighter than fiber is separated into the reject faction. The pulp fed to the hydrocyclone cleaner is divided into two fractions, such as an accept faction and a reject faction. The locations of the outlets for these fractions on the hydrocyclone cleaner are reverse as compared to a conventional hydrocyclone. In reverse centrifugal cleaning, the accept faction is discharged from the bottom of the hydrocyclone cleaner and the reject faction from the top of the cleaner. The feed pulp is thickened into the accept faction, whereby the reject faction is at a lower consistency than the feed pulp and the accept faction is at a higher consistency than the feed pulp.
In building and connecting a reverse centrifugal cleaning plant, the use of a conventional process, such as presented in U.S. Pat. No. 6,003,683, is known. In such conventional processes, a reverse centrifugal cleaning plant is constructed such that the first stage is provided with a so-called reverse hydrocyclone cleaner and the second stage with so-called three-way cleaners. A three-way cleaner is not a reverse hydrocyclone cleaner, but it is mainly a combination of a conventional and a reverse hydrocyclone cleaner. In a three-way cleaner, the reject is taken at a low location from the center of the cleaner axially and the accept is taken at a low position from the outer wall of the cleaner tangentially. The use of a three-way cleaner is based on the possibility to take out remarkably less reject than from a reverse hydrocyclone cleaner, whereby the total reject flow of the plant remains low. Additionally, the pressure difference applied in a three-way cleaner is considerably smaller than in a reverse hydrocyclone cleaner, whereby it is more energy-efficient. On the other hand, the separation efficiency of a three-way cleaner for particles lighter than fiber and water is lower.
In a conventional reverse centrifugal cleaning plant used by e.g. KBC (Kadant Black Clawson), the accept from the first stage is led further to a thickener and dilute reject is fed into a second stage. This means that the feed consistency of the second stage is very low. From the second stage the accept is led further into a dilution water or white-water tank and the reject is led to a clarifier. This kind of solution is presented e.g. in publication WO 97/06871.
Another solution commonly used in conventional reverse centrifugal cleaning plants is to use cascade connection. For instance GL&V (Groupe Laperrière & Verreault Inc.) builds a reverse centrifugal cleaning plant using cascade connection and in both stages reverse hydrocyclones.
In a conventional reverse centrifugal cleaning plant of GL&V, the accept from the first stage is led further to a thickener and dilute reject is fed into a second stage. This means that the feed consistency of the second stage is very low in this solution. From the second stage the accept is led back into the first stage feed (so-called cascade-connected system) and the reject is led to a clarifier. A solution of this type is presented e.g. in publication WO 98/11296.
The use of prior art reverse vortex cleaning plants involves the problem of low separation efficiency of the second stage or stages after that. When studying the separation efficiency of a reverse hydrocyclone cleaner for impurities lighter than fiber and water, such as wax, we (the inventors) noticed that the separation efficiency is highly dependent on the consistency of the feed suspension. It has earlier been thought that a hydrocyclone cleaner operates efficiently only at a consistency low enough.